The present invention relates to optical fiber transmission systems. More specifically, the present invention relates to methods and apparatus for reducing Raman crosstalk in a wavelength-division-multiplexing (WDM) optical fiber transmission system.
Optical fibers offer the highest capacity of any communication medium. One example of an optical fiber transmission system which has become prevalent is the Passive Optical Network (PON). In a PON, signals are carried over optical fibers using lasers, without the need for active electronics. PONs provide significant increases in network capacity, as well as significant savings in implementation costs since the use of active electronics is avoided.
International Telecommunications Standard (ITU) G.983.1 describes an Asynchronous Transfer Mode (ATM) based Passive Optical Network (PON), commonly referred to as a Broadband PON or BPON. ITU G.983.3 provides a Wavelength-division-multiplexing (WDM) enhancement to ITU G.983.1. This WDM enhancement provides a video service capability to the PON. Since video is typically a broadcast service, it is advantageous to provide a broadcast channel on the PON for broadcast video service. In ITU G.983.3, this service is provided using WDM.
WDM is an essential technology used to channelize the optical fiber. Because of nonlinear effects in the fiber, each WDM channel is potentially impaired by other WDM channels.
In WDM optical fiber transmission systems in general, and in the system described in ITU G.983.3 in particular, there is a problem with crosstalk caused by Raman interaction between the optical wavelengths. This problem is well known in the literature.
One of the important characteristics of the Raman effect is that the crosstalk is much more pronounced at low frequencies, and is attenuated at the high frequencies. Raman crosstalk is a serious problem in that it can limit the performance of the transmission system, represented as a bit error rate floor for digital signals, or a limit on the carrier-to-noise ratio for analog signals.
It would be advantageous to provide simple methods and apparatus for reducing Raman crosstalk in WDM optical fiber transmission systems. It would be further advantageous to provide such Raman crosstalk reduction in a simple manner by controlling the idle data pattern of idle data used to maintain the optical link.
The methods and apparatus of the present invention provide the foregoing and other advantages.